React Native Dimensions is only a partial answer to this question, I came here looking for the actual pixel size of the screen, and the Dimensions actually gives you density independent layout size.
You can use React Native Pixel Ratio to get the actual pixel size of the screen.
You need the import statement for both Dimenions and PixelRatio
import { Dimensions, PixelRatio } from 'react-native';
You can use object destructuring to create width and height globals or put it in stylesheets as others suggest, but beware this won't update on device reorientation.
const { width, height } = Dimensions.get('window');
From React Native Dimension Docs:
Note: Although dimensions are available immediately, they may change (e.g due to >device rotation) so any rendering logic or styles that depend on these constants >should try to call this function on every render, rather than caching the value >(for example, using inline styles rather than setting a value in a StyleSheet).
PixelRatio Docs link for those who are curious, but not much more there.
To actually get the screen size use:
PixelRatio.getPixelSizeForLayoutSize(width);
or if you don't want width and height to be globals you can use it anywhere like this
PixelRatio.getPixelSizeForLayoutSize(Dimensions.get('window').width);